Advances in laser source technology and in optical recording systems have brought about a great demand for a reliable optical recording media which is suitable as a data storage device. To be suitable as a data storage device, the optical recording media should have high sensitivity to a laser for recording data; good signal to noise ratio on playback; no degradation on repeated playback; durability under normal operating conditions; a resolution of one micron or better; a low bit error rate; and manufactureability at low cost.
Known optical recording medias have been made of tellurium films. Although tellurium films have adequate sensitivity to a laser for recording data, these films degrade rapidly under normal operating conditions. In order to prolong the life of tellurium optical recording medias, alloys of tellurium have been developed. Tellurium alloys, however, have been found to be insufficiently sensitive to a laser for recording data. Tellurium films have also been enclosed in an inert gas sandwich in order to increase its durability. However, the cost of making an optical recording disc from the sandwiched tellurium is prohibitively expensive.
In a copending U. S. patent application, Ser. No. 771,252, filed Aug. 30, 1985, an optical recording media is disclosed which overcomes the above problems. The optical recording media is formed by coating a substrate with a metal film; oxidizing a layer of the metal film; and depositing a dielectric-like film on the oxidized layer. When exposed to a focused laser, the coloration of the dielectric-like film changes to store data. Although this media is sensitive to a laser for recording data when the laser's pulse length is very short, that is, on the order of a few nanoseconds, when the laser's pulse length is longer, heat conduction from the metal film coating the substrate degrades the media's sensitivity.
Multi-layer optical recording medias have been proposed which include a spacing film disposed between two metal films wherein the spacing film has low heat conductivity. However, in these medias the thickness of not only the top metal film, but of the spacing layer is critical for recording. The thickness of the spacing layer must be an a (2N+1) multiple of the quarter wavelength of the laser light used in recording wherein N is 0 or a positive integer. Any variations in the thickness of either the top metal film or the spacing film will result in variations in the recording sensitivity of the media. Further, when such spacing films are used, the top metal film is made very thin such as on the order of 50 angstroms resulting in an optical recording media which has poor durability and a high error rate.